This application claims the priority of Japanese Patent Applications No.11-163364 filed on Jun. 10, 1999 filed which is incorporated here in by reference.
The present invention relates to an automatic transmission which comprises a power transmission mechanism incorporating a plurality of power transmission paths and a plurality of hydraulically operated frictionally engaging means. These hydraulically operated frictionally engaging means are controlled to select these power transmission paths individually, by means of hydraulic oil which is supplied through a plurality of shift control valves.
Such automatic transmissions have been known and are utilized, for example, as automatic transmissions for vehicles. Typically, an automatic transmission designed for use in a vehicle operates such that the transmission automatically controls the actuation of hydraulic clutches to change the speed change ratio in correspondence to the driving condition of the vehicle. Generally, the automatic transmission includes a speed change hydraulic unit which comprises a plurality of shift control valves, a solenoid valve to control the actuation of these shift control valves, and a manual valve operated in correspondence to the movement of the shift lever manipulated by a driver. In this arrangement, the automatic transmission automatically performs speed change control for a plurality of ranges, i.e., a reverse drive range, a neutral range and a forward drive range (D range, Second, First, etc.) which are individually selected by the operation of the manual valve operated in correspondence to the manipulation of the shift lever (generally, this automatic control is effective only in the forward drive range).
Recently, another type of automatic transmission has been introduced. This automatic transmission does not use a manual valve for the selection of a range and controls the shift of speed ratios in each range solely by electrical signals. Such transmissions are disclosed in Japanese Laid-Open Patent Publication Nos. H5 (1993)-209683 and H5 (1993)-215228. Each automatic transmission disclosed there comprises a plurality of solenoid valves to control the actuation of shift control valves. The selection of the ranges, i.e., the forward drive range, the neutral range, and the reverse drive range, as well as the selection of the speed ratios in the forward drive range are performed in correspondence to the command signals (electrical signals) which actuate these solenoid valves.
As such a system controls the selection of drive ranges and speed ratios solely by the hydraulic pressure, there is a concern that the drive range selection may be performed inaccurately when a solenoid valve breaks down or malfunctions. To improve the performance of the system, Japanese Laid-Open Patent Publication No. H5 (1993)-223156 discloses a system which comprises oil pressure sensors and range determining means. These oil pressure sensors detect the hydraulic pressure generated for establishing the drive ranges, and the range determining means determines, from the actuation signals of the solenoid valves, in which drive range the transmission is set currently. In this construction, the system compares the drive range determined by the range determining means with the drive range actually set, the latter being detected by the oil pressure sensors, to judge whether the transmission is controlled correctly. If an abnormality is detected, then the system uses extra valves through which the hydraulic pressure is supplied to a plurality of oil passages for the control of the transmission.
This system requires a number of oil pressure sensors, each sensor for detecting the hydraulic pressure generated for establishing a corresponding drive range. If one of these sensors breaks down, then the detection of the drive range may be affected, so the detection of malfunctions will become unreliable. Moreover, because the system needs to establish a plurality of oil passages when a breakdown is detected, the hydraulic circuit of the system is complex, which often results in an increased production cost.
It is an object of the present invention to provide a control system for an automatic transmission, which system is relatively simple in construction and is capable of performing appropriate speed change control in response to a manipulation of drive range switching even in an event where some elements experience failures.
To achieve this objective, the present invention provides a control system for an automatic transmission comprising a power transmission mechanism (for example, the parallel shaft speed change mechanism TM of the embodiment described in the following section), a plurality of hydraulically operated frictionally engaging means (for example, the LOW clutch 11, the SECOND speed clutch 12, the THIRD speed clutch 13, the FOURTH speed clutch 14 and the FIFTH speed clutch 15 of the following embodiment), a plurality of shift control valves (for example, the first shift valve 60 or 260, the second shift valve 62 or 262, the third shift valve 64 or 264, the fourth shift valve 66 or 266, the fifth shift valve 68 or 268 and the CPB valve 56 of the following embodiment), a forward/reverse hydraulic servomechanism (for example, the forward/reverse selection hydraulic servomechanism 70 of the following embodiment), a D inhibitor valve and a plurality of solenoid valves (for example, the firstxcx9cfifth on/off solenoid valves 81xcx9c85 and the firstxcx9cfourth on/off solenoid valves 281xcx9c284 of the following embodiments). The power transmission mechanism incorporates a plurality of power transmission paths, and the hydraulically operated frictionally engaging means are provided inside the power transmission mechanism and are used for selecting the power transmission paths individually. The shift control valves are used for controlling the supply of hydraulic oil to the hydraulically operated frictionally engaging means, and the forward/reverse hydraulic servomechanism is used for switching and selecting either the power transmission paths assigned for a forward drive range or the power transmission paths assigned for a reverse drive range. The D inhibitor valve is switched for controlling the supply of a line pressure into the forward drive and reverse drive servo-oil chambers of the forward/reverse hydraulic servomechanism, and the plurality of solenoid valves are used for controlling the actuation of the shift control valves and the D inhibitor valve. Specifically, the D inhibitor valve is switchable between a forward drive position which connects the reverse drive servo-oil chamber to a drain and a reverse drive position which allows the line pressure into the reverse drive servo-oil chamber. Furthermore, the control system comprises a first neutral mode and a second neutral mode as neutral modes to be set when a manipulation is made to shift the transmission to a neutral range. In the first neutral mode, the D inhibitor valve is switched to the forward drive position, and the forward drive servo-oil chamber is connected to the drain through one of the shift control valves. In the second neutral mode, the D inhibitor valve is switched to the reverse drive position, and the forward drive and reverse drive servo-oil chambers are connected to the drain through one of the shift control valves.
In the first neutral mode, because the D inhibitor valve is positioned at the forward drive position, and the reverse drive servo-chamber is drained, there is no possibility that the forward/reverse hydraulic servomechanism be accidentally switched to an R range position to select the power transmission path assigned for the reverse drive range in an event that a shift control valve malfunctions. In a similar way, in the second neutral mode, because the D inhibitor valve is positioned at the reverse drive position, there is no possibility that the forward/reverse hydraulic servomechanism be accidentally switched to the D range position to select the power transmission paths assigned for the forward drive range in an event that a solenoid valve or a shift control valve malfunctions. The control system sets the first or second neutral mode appropriately for the neutral range in correspondence to a type of shift manipulation performed by the driver. In an event that a shift control valve or a solenoid valve malfunctions, the system fixes the forward/reverse hydraulic servomechanism at either the D range position or the R range position and thereby prevents the forward/reverse hydraulic servomechanism from being switched accidentally.
For example, when a manipulation is made to shift the transmission from the forward drive range through the neutral range to the reverse drive range, the control system sets the second neutral mode to switch the D inhibitor valve to the reverse drive position and then, keeping the D inhibitor valve at the reverse drive position, sets a reverse mode in which the line pressure is supplied into the reverse drive servo-chamber through one of the shift control valves. In this way, for example, if there is a breakdown in the control of the D inhibitor valve, the system can retain the forward/reverse hydraulic servomechanism as is and prevents it from being switched accidentally.
In this case, it is preferable that the solenoid valve having been used for switching the D inhibitor valve to the reverse drive position for setting the second neutral mode be turned on or off into an opposite state to supply the line pressure into the reverse drive servo-oil chamber for setting the reverse drive mode. If this solenoid valve experiences a breakdown, then the D inhibitor valve or the forward/reverse hydraulic servomechanism can be fixed to prevent the forward/reverse hydraulic servomechanism from being switched accidentally.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.